This invention relates to a phototropic, light weight, highly refractive spectacle glass with a low dispersion.
Phototropic glasses are nowadays used predominantly in spectacles and are also finding increasing popularity in corrective prescription lenses for defective eyesight. For the correction of defective eyesight with glass which has a refractive index of 1.523, the spectacle lenses become progressively more voluminous at higher diopters (more serious sight defects); this means greater glass weight. For this reason, highly refractive but at the same time relatively light-weight lens-glasses have been sought and found; but every attempt at producing such lens-glasses also having good phototropic properties has so far failed to give the desired result. The reason for this lies in the composition of the highly refractive light-weight lens-glasses which does not enable the achievement of optimal precipitation of silver halide-containing zones in the glass by appropriate annealing conditions. This is essential for good phototropism. For silver halide-based phototropic glasses this phase separation is indispensable. Its quality determines the quality of phototropism.
If, for example, the conventional highly refractive, light weight glasses according to DE-PS 2 259 183 are melted with phototropic carriers, the result will be a white opaque substance which cannot be used for making spectacle glass.
The current state of the art is well represented in the field of highly refractive light-weight glasses by German No. PS 2 259 183 (U.S. Pat. No. 3,898,093), and in the field of phototropic glasses by DE-PS No. 2 404 752 (U.S. Pat. No. 4,108,674) and DE-PS No. 2 223 629(U.S. Pat No. 4,149,896).
Highly refractive phototropic glasses are likewise known but because of their heavy weight, they are of no interest for the purposes of the present invention.
A general review of conventional silver halide-containing phototropic glasses, insofar as they have properties which deviate from the standard refractive index for spectacle glasses of n.sub.d =1.523, has revealed that it would seem impossible to produce an SiO.sub.2 - containing phototropic glass with a refractive index equal to or greater than 1.59, an Abbe number equal to or greater than 40 and a density equal to or less than 3.2 g/cm.sup.3. It is true that DE-OS No. 2 140 915 describes glasses with a relatively high refractive index, but these glasses must not contain SiO.sub.2 because otherwise the product becomes opaque.
If this result is applied to the glasses according to DE-PS No. 2 259 183, the essential problem of producing highly refractive light-weight SiO.sub.2 -glasses with phototropic characteristics becomes readily apparent: the more highly refractive light-weight SiO.sub.2 -glasses, provided with the components silver and halogens, as well as phototropic highly refractive glasses, provided with SiO.sub.2 for the purpose of stabilization and suitable for mass production, become clouded and eventually opaque.
The glasses according to DE-OS No. 2 140 915 moreover have density values over 3.2 g/cm.sup.3. A review of the glasses according to DE-OS No. 2 256 775 revealed only glasses with refractive indices smaller than 1.59. A review of the glasses according to German OS No. 2 260 879 only showed glasses with densities higher than 3.2 g/cm.sup.3.
A combination of the criteria: phototropism, low density, low dispersion and high refractive index, was achieved for the first time in DE AS No. 3 117 000.5 (U.S. Pat. No. 4,486,541), which describes a phototropic glass with the optical characteristics n.sub.d greater than or equal to 1.59, an Abbe number greater than or equal to 40 and a density less than or equal to 3.2 g/cm.sup.3. However, this glass is still deficient, especially as regards its density, lower values being needed.
The Abbe number ##EQU1## is significant inasmuch as in the region of values less than 40, it is the cause of interfering color fringes appearing when the angle of view through the glasses is at an angle. This phenomenon is due to unduly high dispersion, i.e., excessively high wavelength dependency of the refractive index. Such color fringes must be avoided. Any further increase in the Abbe number yields an improvement in dispersion and thus in serviceability.
Another important aspect resides in practicability under conventional modern production conditions. Some raw materials, such as, for example, tantalum oxide, are so expensive nowadays that their use for the production of spectacle glasses is severely restricted even though, due to their effect on network formation, i.e., for the extremely important devitrification stability, they are of the greatest interest in the production process. Added to this is the fact that, for reasons of mass production in tank furnaces and automatic presses (viscosity and crystallization criteria), and SiO.sub.2 content of at least 30% by weight is indispensible for this type of spectacle glass.